Medical sensor with compressible light barrier and technique for using the same

ABSTRACT

According to various embodiments, a medical sensor assembly may include compressible light barriers configured to prevent undesired light from being detected. The compressible light barriers may protrude from the surface of the sensor. However, when applied to the tissue, the compressible light barriers may be compressed to the point of being substantially flush with the tissue.

CROSS-REFERENCE TO RELATED APPLICATIONS

This claims the benefit of U.S. Provisional patent Application No.61/163,358, filed Mar. 25, 2009, which is hereby incorporated byreference herein in its entirety.

BACKGROUND

The present disclosure relates generally to medical devices and, moreparticularly, to sensors used for sensing physiological parameters of apatient.

This section is intended to introduce the reader to aspects of the artthat may be related to various aspects of the present disclosure, whichare described and/or claimed below. This discussion is believed to behelpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentdisclosure. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

In the field of medicine, doctors often desire to monitor certainphysiological characteristics of their patients. Accordingly, a widevariety of devices have been developed for monitoring many suchphysiological characteristics. Such devices provide doctors and otherhealthcare personnel with the information they need to provide the bestpossible healthcare for their patients. As a result, such monitoringdevices have become an indispensable part of modern medicine.

One technique for monitoring certain physiological characteristics of apatient is commonly referred to as pulse oximetry, and the devices builtbased upon pulse oximetry techniques are commonly referred to as pulseoximeters. Pulse oximetry may be used to measure various blood flowcharacteristics, such as the blood-oxygen saturation of hemoglobin inarterial blood, the volume of individual blood pulsations supplying thetissue, and/or the rate of blood pulsations corresponding to eachheartbeat of a patient. In fact, the “pulse” in pulse oximetry refers tothe time varying amount of arterial blood in the tissue during eachcardiac cycle.

Pulse oximeters typically utilize a non-invasive sensor that transmitslight through a patient's tissue and that photoelectrically detects theabsorption and/or scattering of the transmitted light in such tissue.One or more of the above physiological characteristics may then becalculated based upon the amount of light absorbed and/or scattered.More specifically, the light passed through the tissue is typicallyselected to be of one or more wavelengths that may be absorbed and/orscattered by the blood in an amount correlative to the amount of theblood constituent present in the blood. The amount of light absorbedand/or scattered may then be used to estimate the amount of bloodconstituent in the tissue using various algorithms.

Pulse oximetry readings involve placement of a sensor on a patient'stissue, typically via a lightly adhesive sensor, a clip-style sensor, ora sensor that may be fitted into a wearable garment, such as a hat or aheadband. If the hat or headband is not closely fitted to the patient'stissue, ambient light may interfere with the sensor's light detection.Some outside light infiltration into the sensor may be avoided byfitting the sensor snugly against the patient's tissue. However, such aconforming fit may be difficult to achieve over a range of patientphysiologies without adjustment or excessive attention on the part ofmedical personnel. Additionally, an overly tight fit may cause localexsanguination of the tissue around the sensor. Exsanguinated tissue,which is devoid of blood, may shunt the sensor light through the tissue,which may also result in increased measurement errors.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the disclosure may become apparent upon reading thefollowing detailed description and upon reference to the drawings inwhich:

FIG. 1A illustrates a perspective view of a hat structure for holding apulse oximetry sensor on a patient's tissue according to an embodiment;

FIG. 1B illustrates a perspective view of an exemplary pulse oximetrysensor body with an compressible light barrier that may be incorporatedwith the hat of FIG. 1A;

FIG. 1C illustrates a cross-sectional view of the pulse oximetry sensorbody taken along line 1C-1C of FIG. 1B;

FIG. 1D illustrates a cutaway view of the hat of FIG. 1A with the pulseoximetry sensor with a compressible light barrier as shown in FIG. 1Band FIG. 1C;

FIG. 2 illustrates a perspective view of a forehead sensor with acompressible light barrier arranged in concentric circles according toan embodiment;

FIG. 3 illustrates a cross-sectional view of the sensor of FIG. 2 takenalong line 3-3;

FIG. 4 illustrates a cross-sectional of a sensor with a compressiblelight barrier with light reflective and light absorptive surfacesaccording to an embodiment;

FIG. 5 illustrates a perspective view of a headband-style sensor with acompressible light barrier according to an embodiment;

FIG. 6 illustrates a pulse oximetry system coupled to a multi-parameterpatient monitor and a sensor according to an embodiment; and

FIG. 7 is a block diagram of a pulse oximetry system according to anembodiment.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

One or more specific embodiments of the present disclosure will bedescribed below. In an effort to provide a concise description of theseembodiments, not all features of an actual implementation are describedin the specification. It should be appreciated that in the developmentof any such actual implementation, as in any engineering or designproject, numerous implementation-specific decisions must be made toachieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

Optical medical sensors are provided that reduce the amount of outsidelight that may impinge the detecting elements of a sensor. Such sensorsmay also reduce the amount of “shunted” light, i.e., light originatingfrom light emitting elements of the sensor that impinges the detectingelements of a sensor without first passing through tissue. Such sensorsmay incorporate surface features on the tissue-contacting surface of thesensor to influence the path of light from the undesired light sourcesand/or to direct such light away from the detecting elements of thesensor. Such sensors may absorb or reflect the light originating fromthese undesired light sources before such light can impinge thedetecting elements of the sensor.

Pulse oximetry sensors are typically placed on a patient in a locationthat is normally perfused with arterial blood to facilitate measurementof the desired blood characteristics, such as arterial oxygen saturationmeasurement (SpO₂). The most common sensor sites include a patient'sfingertips, toes, earlobes, or forehead. Regardless of the placement ofa sensor used for pulse oximetry, the reliability of the pulse oximetrymeasurement is related to the accurate detection of transmitted lightthat has passed through the perfused tissue and that has not beensupplemented by undesired light sources. Such supplementation and/ormodulation of the signals transmitted to a monitor by the sensor cancause variability in the resulting pulse oximetry measurements. Thecontribution of ambient and/or shunted light may affect the measurementof the particular blood constituent, such as SpO₂.

In many cases, light from undesired light sources propagates along anoptical path that is distinguishable from the optical path of the lightthat is related to a blood constituent. In a transmission-type sensor,the sensor's emitter and detector lie on opposing sides of the tissuewhen the sensor is applied to a patient. The optical path of the signallight, which is light originating from the emitter that properly passesthrough perfused tissue, is substantially in-line with an imaginary axisconnecting the emitter and the detector. For reflectance-type sensors,the optical path of the emitted signal light is somewhat morecomplicated, as the light first enters the perfused tissue and then isscattered back to the detector. In both transmission-type andreflectance-type sensors, shunted light and ambient light generallypropagate at angles substantially off-axis from the optical path of thesignal light.

The sensors discussed below have compressible light barriers that act todivert shunted and/or ambient light away from the light detectingelements of a sensor. In an embodiment, an oximetry sensor with suchcompressible light barriers may be adapted for placement in a hat (forexample, a neonatal stocking cap), a headband, or other wearablestructure (i.e. a glove, a sock, a wristband) to apply the sensor on thebody of the user. FIGS. 1A-1D illustrate an assembly drawing of anembodiment of a sensor assembly 10 including a wearable structure, whichmay be a hat 11, as shown in FIG. 1A. A reflectance-type pulse oximetrysensor 15, as shown in FIG. 1B, is adapted to be placed or adhered tothe inside of the hat 11. The sensor 15 may include a substrate 14 thatmay be made from any suitable material. In an embodiment, the substrate14 is a foam or other conformable material. In one embodiment, thesubstrate 14 is black or dark in color to absorb stray light and furtherminimize any shunting of light between sensor and patient skin. Thesubstrate 14 may include an adhesive material to secure the sensordirectly to the tissue. In one embodiment, the sensor 15 may include anemitter 16 containing emitters for two or more wavelengths of lights anda detector 18 spaced apart from the emitter 16. The sensor 15 also mayinclude compressible light barrier 12, discussed in more detail below,configured to block undesired light from reaching the detector 18.

Also shown in FIG. 1B is a cable 20 for providing drive current to theLED component of the emitter 16, and providing the detector signal tothe medical device. In addition to providing the electrical connectionto the downstream medical device, the cable 20 may provide shielding toprotect the signals from the detector against external electricalinterference. In addition, the sensor 15 may include suitable structuresfor providing electrical connections to the cable 20 and/or downstreammedical device, such as a flex circuit, a Faraday shield, and leadsconnecting the optical components of the sensor 15 to the electricalcomponents.

FIG. 1C is a cross-sectional side view showing the compressible lightbarrier 12 that substantially surrounds emitter 16. The compressiblelight barrier 12 may protrude from the surface 14 of the sensor 15. Whenthe hat 11 is applied to the tissue, the force of the elastic band orknit of the hat 11 may provide sufficient force such that thecompressible light barrier 12 is compressed against the tissue to form aseal. Any light from the emitter 16 on a path, represented by arrow 23,to shunt directly to the detector 18 is blocked by the interior walls 22of the compressible light barrier 12. Accordingly, light from theemitter 16 may be directed towards the tissue and away from the shuntpath to the detector 18, which may improve the accuracy of measurementsfrom the sensor 15.

As shown, the compressible light barrier 12 surrounds the emitter 16.However, in embodiments the compressible light barrier 12 may surroundthe detector 18, or both the emitter 16 and the detector 18, or may bedisposed in a region between the emitter 16 and the detector 18. In anembodiment, it may be advantageous for the footprint of the compressiblelight barrier 12 on the surface 14 to be minimized. For example, whilethe compressible light barrier 12 may act to prevent unwanted light fromreaching the detector, the compressible light barrier 12, because itprotrudes from the surface 14, may also interfere with a conforming fitof the sensor 15 to the tissue. Accordingly, a balance between the lightbarrier properties and the conforming fit may be achieved by providingthe compressible light barrier 12 on less than 25%, less than 20%, lessthan 15%, less than 10%, or less than 5% of the surface 14. This may beachieved by providing the compressible light barrier 12 with relativelythin walls, or by surrounding only one of the emitter 16 and thedetector 18 rather than both. In an embodiment, the compressible lightbarrier 12 surrounding the emitter 16 may have a different, or smaller,shape than the compressible light barrier 12 surrounding the detector18. For example, it may be advantageous to have a smaller compressiblelight barrier 12 surrounding the emitter 16 to more narrowly focus thelight directed towards the tissue. In addition, the compressible lightbarrier 12 may be configured to protrude minimally from the surface 14.The compressible light barrier 12 may protrude from the surface 14 atleast about 0.5 mm, 1 mm, 2 mm, or 5 mm in its uncompressed state. Itshould be understood that the compressible light barrier 12, whencompressed by the force of application of the sensor 15 to the tissue,may protrude only slightly from the surface 14. For example, thecompressible light barrier 12 may protrude from the surface 14 about 0.5mm, 1 mm, 2 mm, in its uncompressed state. A compressible light barrier12 that is 5 mm in height in its uncompressed state may be compressed toa height of 2.5 mm (50% of its uncompressed height) or 1.25 mm (25% ofits uncompressed height). Similarly, a compressible light barrier 12that is 2 mm in height in its uncompressed state may be compressed to aheight of 1 mm (50% of its uncompressed height) or 0.5 mm (25% of itsuncompressed height).

In an embodiment, the compressible light barrier 12 is formed frommaterials with low durometer or a high degree of compressibility, suchas foams, silicone, polyvinyl chloride, or gels, for example, a materialhaving a compression modulus of about 2.0 or less. The compressionmodulus for polyurethane foam is a function of the density of the foamand the structure of the foam. Generally, compression modulus increasesas foam density increases. In embodiments, different chemicalformulations and manufacturing processes may be used to create foamswith different foam cell structures. Foams with high concentration ofclosed cells (closed-cell foam) typically have a higher compressionmodulus than foams with high concentration of open cells (open-cellfoam). In an embodiment, compressibility may be measured by an IndentionLoad Deflection (ILD) test, which measures the load-bearing capacity ofa standard specimen indented by a circular compressor foot of 50 squareinches as 25% deflection. The sample is placed on a perforated plate(perforated plate has ¼″ holes on ¾″ centers) and is deflected twice to25% of its original height and then allowed to relax for 10+/− minutes.The height of the foam is then rechecked. The new height is determinedand then the sample is deflected 25%, or to 75% of the crushed height.The foam is then held in this fashion for one minute and the load on thescale is then read. In one embodiment, the ratio (compression modules)of the compressive force needed to indent the foam to 25% and 50%,respectively, of its thickness is determined. The greater the value ofthis ratio, the greater the degree of firmness that is offered by thefoam. Low density foams have a compression modulus of around 1.20. Inembodiments, the compression modulus of the compressible light barrier12 may be less than about 2.0, less than about 1.5 or less than about1.2.

The sensor assembly 10 is shown fully assembled in FIG. 1D. As shown,the sensor 15 is positioned on the interior of the hat 11 such that theemitter 16 and detector 18 may come into contact with the skin when thesensor assembly 10 is applied to a patient. The sensor 15 may beattached (e.g., adhered or sewn into) to the inside band of the hat 11.In one embodiment, the hat 11 may include indicators to position thesensor 15 on a particular location on the patient's forehead, forexample to position the sensor 15 on the lower forehead region, abovethe eyebrow, with the sensor optics (emitter 16 and detector 18) locatedabove and predominantly lateral to or centered over the iris. Thelocation of the reflectance sensor 15 in the hat allows appropriateplacement of the sensor in the desired forehead location by a user. FIG.1D shows that the cable 20 is positioned through a hole in the top ofthe hat 11. In an embodiment, the cable 20 may be adhered or otherwiseconstrained in the hat 11 so that the cable 20 generally is positionedaway from the sensor 15 to avoid interfering with the patient's eyesightor bothering the patient.

In an embodiment, the force of the sensor 15 against the tissue,including any force provided by the compressible light barrier 12, maynot exsanguinate the tissue. In one embodiment, it is contemplated thatthe force that hat 11 (or other wearable assembly, such as a headband orclip-style sensor) exerts on the tissue sufficient pressure so that thepressure exceeds the typical venous pressure of a patient, but does notexceed the diastolic arterial pressure. As the pulse oximetrymeasurements are related to arterial blood oxygen saturation andpulsation, and not venous blood pulsation, reducing the effect of thevenous component in the tissue may enhance the sensitivity of the sensorto variations in the arterial blood signal. Thus, the sensor assembly 10may apply a pressure greater than the venous pressure to squeeze excesspooled venous blood from the optically probed tissue. Yet, since thepressure applied by the sensor assembly 10 is designed to be less thanthe arterial pressure, the application of pressure to the tissue doesnot interfere with the arterial pulse signal. Typical venous pressure,diastolic arterial pressure, and systolic arterial pressure are lessthan 10-35 mmHg, 80 mmHg, and 120 mmHg, respectively. Accordingly, incertain embodiments, the sensor assembly 10 may be adjusted to overcomean average venous pressure of 15-35 mmHg. However, venous pressures mayvary because of the location of the vascular bed and the patient'scondition. For example, low arterial diastolic blood pressure (about 30mmHg) may occur in sick patients. In such embodiments, the sensorassembly 10 removes most of the venous pooling with by applyingsufficient pressure to overcome light to moderate venous pressure (about15 mmHg).

In one embodiment, the force applied to the tissue to overcome thevenous pressure may be sufficient to compress the compressible lightbarrier 12 to an appropriate degree, for example to at least 50% of itsoriginal uncompressed height or at least 25% of its originaluncompressed height, so that the light barrier properties remain intact.In another embodiment, if the force applied to the sensor is increased,the compressible light barrier 12 may act as a pressure absorber,preventing the skin from becoming exsanguinated in the case of aninappropriately high pressure. In such an embodiment, the compressiblelight barrier 12 may then be compressed to an even greater degree, sothat it lies substantially flush with the surface 14.

It should be appreciated that the compressible light barrier 12 may bearranged in any suitable manner on the surface of the sensor 15 toprevent ambient or shunted light from reaching the detector 18. In anembodiment, shown in FIG. 2 and FIG. 3, compressible light barrier 12may be arranged to surround the emitter 16 and the detector 18. FIG. 2is a perspective view of an exemplary forehead sensor 24 with acompressible light barrier 12 that includes concentric circles 26 thatsubstantially surround an emitter 16 and a detector 18, and FIG. 3 is across-sectional view of the sensor 24 applied to a patient's forehead28. Such an arrangement of concentric circles 26 may be advantageous informing a seal with the tissue 28, thus creating a barrier against anyambient light or shunted light that may leak into the sensor 24. Theambient light, depicted by wavy arrows 30, impinges the compressiblelight barrier 12 and is prevented from reaching the detector 18. Theoptical path of the signal light, depicted by wavy arrow 32, issubstantially unaffected by the compressible light barrier 12.

In one embodiment, the compressible light barrier 12 may include lightabsorbing materials, light reflecting materials, light refractingmaterials, or any combination thereof. For example, a surface, includingall or part of a compressible light barrier 12, may be formed from,coated with, or impregnated with such materials. It should also beappreciated that, as discussed above, the compressible light barrier 12may contain such materials only on a tissue-contacting surface, or, inalternate embodiments, the sensor body may be constructed entirely fromsuch materials in appropriate regions as described herein. As shown inFIG. 4, a cross-sectional view of a sensor 32, the compressible lightbarrier 12 surrounding the emitter 16 may include a light reflectivesurface 34 and a light absorptive surface 36. As shown, the lightsurface 34 may face the emitter 16, while the light absorptive surface36 may be surfaces that do not face the emitter 16. Such an arrangementmay allow off-angle light from the emitter 16 to be reflected backtowards the tissue, which may help to increase the signal intensityreceived at the detector 18. In addition, dark or absorptive surfaces onother areas of the compressible light barrier 12 may absorb ambientlight that leaks from the edges of the sensor 32. Similarly, thecompressible light barrier 12 surrounding the detector 18 may include alight reflective surface 34 to help redirect light towards the detector18 while absorptive surface 36 may absorb shunted or ambient lightbefore it reaches the detector 18. Examples of light absorbing materialsmay include, but are not limited to, black or dark pigment, black ordark woven fabric or cloth, and infrared blockers. Examples of suitablelight reflecting materials include white or silver pigment, metals, ormirrored surfaces. Further, the surface 40 of the sensor 32 may be lightreflective or light absorptive.

It should also be appreciated that light absorbing materials may beadapted to absorb light at a particular wavelength. In certainembodiments, when light absorbing material is disposed between anemitter and a detector of a sensor, it may be advantageous to use lightabsorbing material that absorbs a wavelength emitted by the emitter inorder to absorb shunted light from the emitter. For example, a lightabsorbing material may absorb at least about 50% of one or morewavelengths of light from the emitter, or may absorb a range of 50% to95% of one or more wavelengths of light from the emitter. A lightabsorbing material may also absorb at least about 90% to at least 95% ofone or more wavelengths of visible light and near-infrared light. In aspecific embodiment, a pulse oximetry sensor may emit at least onewavelength of light in the wavelength range of 500 nm-1000 nm. Forexample, a sensor may emit light and wavelengths of 660 nm and 900 nm,which are wavelengths that may be absorbed by dark pigment.

FIG. 5 illustrates an embodiment of a headband-based sensor assembly 50that includes a medical sensor 44 and with a compressible light barrier12. The headband-based sensor assembly 50 may include a strap or band 52that may be fitted around a patient's forehead tissue to contact thesensor 34 with the tissue. In certain embodiments, the sensor 44 maysend feedback to a downstream monitor through cable 20 relating to oneor more blood or tissue constituents.

A sensor or sensor assembly, illustrated generically as a sensorassembly 10, may be used in conjunction with a pulse oximetry monitor60, as illustrated in FIG. 6. It should be appreciated that the cable 20of the sensor assembly 10 may be coupled to the monitor 60 or it may becoupled to a transmission device to facilitate wireless transmissionbetween the sensor assembly 10 and the monitor 60. The monitor 60 may beany suitable pulse oximeter, such as those available from NellcorPuritan Bennett LLC. Furthermore, to upgrade conventional pulse oximetryprovided by the monitor 60 to provide additional functions, the monitor60 may be coupled to a multi-parameter patient monitor 62 via a cable 64connected to a sensor input port or via a cable 66 connected to adigital communication port.

FIG. 7 is a block diagram of an embodiment of a pulse oximeter 60 thatmay be configured to implement the embodiments of the presentdisclosure. Light from emitter 16 may pass into a blood perfused tissue,and may be scattered, and then detected by detector 18. A sensorassembly 10 containing an emitter 16 and a detector 18 may also containan encoder 70 which may be capable of providing signals indicative ofthe wavelength(s) of light source 16 to allow the oximeter to selectappropriate calibration coefficients for calculating oxygen saturation.The encoder 70 may, in an embodiment, be a resistor.

In an embodiment, the sensor assembly 10 may be connected to a pulseoximetry monitor 60. The monitor 60 may include a microprocessor 72coupled to an internal bus 74. Also connected to the bus may be a RAMmemory 76 and a display 78. A time processing unit (TPU) 80 may providetiming control signals to light drive circuitry 82, which controls whenthe emitter 16 is activated, and if multiple light sources are used, themultiplexed timing for the different light sources. TPU 80 may alsocontrol the gating-in of signals from detector 18 through an amplifier83 and a switching circuit 84. These signals are sampled at the propertime, depending at least in part upon which of multiple light sources isactivated, if multiple light sources are used. The received signal fromthe detector 18 may be passed through an amplifier 86, a low pass filter88, and an analog-to-digital converter 90. The digital data may then bestored in a queued serial module (QSM) 92, for later downloading to RAM76 or ROM 96 as QSM 92 fills up.

In an embodiment, based at least in part upon the received signalscorresponding to the light received by detector 18, microprocessor 72may calculate the oxygen saturation using various algorithms. Thesealgorithms may require coefficients, which may be empiricallydetermined, and may correspond to the wavelengths of light used. Thealgorithms may be stored in a ROM 96 and accessed and operated accordingto microprocessor 72 instructions. For example, the encoder 70 maycommunicate with decoder 71 to allow the microprocessor 72 to determinethe appropriate coefficients.

In an embodiment of a two-wavelength system, the particular set ofcoefficients chosen for any pair of wavelength spectra may be determinedby a value indicated by the encoder 70 corresponding to a particularlight source in a particular sensor assembly 10. In one embodiment,multiple resistor values may be assigned to select different sets ofcoefficients, or the sets of coefficients may be stored on a digitalmedium. In another embodiment, the resistors are used to select fromamong the coefficients appropriate for an infrared source paired witheither a near red source or far red source. The selection betweenwhether the near red or far red set will be chosen can be selected witha control input from control inputs 94. Control inputs 94 may be, forinstance, a switch on the pulse oximeter, a keyboard, or a portproviding instructions from a remote host computer. Furthermore, anynumber of methods or algorithms may be used to determine a patient'spulse rate, oxygen saturation or any other desired physiologicalparameter.

The sensor assembly 10 includes an emitter 16 and a detector 18 that maybe of any suitable type. For example, the emitter 16 may be one or morelight emitting diodes adapted to transmit one or more wavelengths oflight in the red to infrared range, and the detector 18 may one or morephotodetectors selected to receive light in the range or ranges emittedfrom the emitter 16. Alternatively, an emitter 16 may also be a laserdiode or a vertical cavity surface emitting laser (VCSEL). An emitter 16and detector 18 may also include optical fiber sensing elements. Anemitter 16 may include a broadband or “white light” source, in whichcase the detector could include any of a variety of elements forselecting specific wavelengths, such as reflective or refractiveelements or interferometers. These kinds of emitters and/or detectorswould typically be coupled to the rigid or rigidified sensor via fiberoptics. Alternatively, a sensor assembly 10 may sense light detectedfrom the tissue is at a different wavelength from the light emitted intothe tissue. Such sensors may be adapted to sense fluorescence,phosphorescence, Raman scattering, Rayleigh scattering and multi-photonevents or photoacoustic effects.

For pulse oximetry applications using either transmission or reflectancetype sensors the oxygen saturation of the patient's arterial blood maybe determined using two or more wavelengths of light, most commonly redand near infrared wavelengths. Similarly, in other applications, atissue water fraction (or other body fluid related metric) or aconcentration of one or more biochemical components in an aqueousenvironment may be measured using two or more wavelengths of light, mostcommonly near infrared wavelengths between about 1,000 nm to about 2,500nm. It should be understood that, as used herein, the term “light” mayrefer to one or more of ultrasound, radio, microwave, millimeter wave,infrared, visible, ultraviolet, gamma ray or X-ray electromagneticradiation, and may also include any wavelength within the radio,microwave, infrared, visible, ultraviolet, or X-ray spectra.

The emitter 16 and the detector 18, and the compressible light barrier12, may be disposed on a sensor body, which may be made of any suitablematerial, such as plastic, foam, woven material, or paper.Alternatively, the emitter 16 and the detector 18 may be remotelylocated and optically coupled to the sensor assembly 10 using opticalfibers. In the depicted embodiments, the sensor assembly 10 is coupledto a cable that is responsible for transmitting electrical and/oroptical signals to and from the emitter 16 and detector 18 of the sensorassembly 10. The cable may be permanently coupled to the sensor assembly10, or it may be removably coupled to the sensor assembly 10—the latteralternative being more useful and cost efficient in situations where thesensor assembly 10 is disposable.

The sensor assembly 10 may be a “transmission type” sensor. Transmissiontype sensors include an emitter 16 and detector 18 that are typicallyplaced on opposing sides of the sensor site. If the sensor site is afingertip, for example, the sensor assembly 10 is positioned over thepatient's fingertip such that the emitter 16 and detector 18 lie oneither side of the patient's nail bed. In other words, the sensorassembly 10 is positioned so that the emitter 16 is located on thepatient's fingernail and the detector 18 is located 180° opposite theemitter 16 on the patient's finger pad. During operation, the emitter 16shines one or more wavelengths of light through the patient's fingertipand the light received by the detector 18 is processed to determinevarious physiological characteristics of the patient. In each of theembodiments discussed herein, it should be understood that the locationsof the emitter 16 and the detector 18 may be exchanged. For example, thedetector 18 may be located at the top of the finger and the emitter 16may be located underneath the finger. In either arrangement, the sensorassembly 10 will perform in substantially the same manner.

Reflectance type sensors also operate by emitting light into the tissueand detecting the light that is transmitted and scattered by the tissue.However, reflectance type sensors include an emitter 16 and detector 18that are typically placed on the same side of the sensor site. Forexample, a reflectance type sensor may be placed on a patient'sfingertip or forehead such that the emitter 16 and detector 18 lieside-by-side. Reflectance type sensors detect light photons that arescattered back to the detector 18. A sensor assembly 10 may also be a“transflectance” sensor, such as a sensor that may subtend a portion ofa baby's heel.

While the disclosure may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the embodiments provided hereinare not intended to be limited to the particular forms disclosed.Indeed, the disclosed embodiments may not only be applied tomeasurements of blood oxygen saturation, but these techniques may alsobe utilized for the measurement and/or analysis of other bloodconstituents. For example, using the same, different, or additionalwavelengths, the present techniques may be utilized for the measurementand/or analysis of carboxyhemoglobin, met-hemoglobin, total hemoglobin,fractional hemoglobin, intravascular dyes, and/or water content. Rather,the various embodiments may cover all modifications, equivalents, andalternatives falling within the spirit and scope of the disclosure asdefined by the following appended claims

What is claimed is:
 1. An apparatus comprising: a stocking capconfigured to be applied to a patient's head; a substrate disposedadjacent the stocking cap; a light emitter disposed on the substrate; alight detector disposed on the substrate; a cable disposed on thesubstrate, wherein the cable extends from the substrate through an openportion of the stocking cap configured to be proximate to a top of thestocking cap when the stocking cap is applied to the patient; and acompressible protrusion disposed on the substrate, wherein thecompressible protrusion is configured to substantially surround at leastone of the emitter or the detector, and wherein the compressibleprotrusion is compressed to less than about 50% of its uncompressedheight when the stocking cap is applied to the patient's head.
 2. Theapparatus of claim 1, wherein the compressible protrusion protrudes atleast about 1 mm from the substrate.
 3. The apparatus of claim 1,wherein the compressible protrusion protrudes substantially orthogonallyfrom the substrate.
 4. The sensor, as set forth in claim 1, comprising asecond compressible protrusion that substantially surrounds thedetector.
 5. The sensor, as set forth in claim 1, wherein thecompressible protrusion comprises a light reflecting material on a firstsurface and a light absorbing material on a second surface.
 6. Thesensor, as set forth in claim 5, wherein the first surface faces theemitter and the second surface opposes the emitter.
 7. The sensor, asset forth in claim 1, wherein the compressible protrusion comprises afoam or gel.
 8. The sensor, as set forth in claim 1, wherein thecompressible protrusion is disposed on less than 10% of a surface of thesubstrate.
 9. The apparatus of claim 1, wherein the stocking capcomprises a stocking cap configured to be placed on the head of aneonate.
 10. The apparatus of claim 1, wherein the stocking cap isconfigured to apply a pressure less than about 35 mm Hg to the patient'shead.
 11. The apparatus of claim 1, the compressible protrusion iscompressed to a height of less than 2.5 mm when the stocking cap isapplied to the patient's head.
 12. A pulse oximetry system comprising: apulse oximetry monitor; and a sensor assembly configured to beoperatively coupled to the monitor, the sensor assembly comprising: alight emitter and a light detector disposed on a substrate; acompressible protrusion disposed on the substrate in an area between theemitter and the detector, wherein the compressible protrusion does notsubstantially exsanguinate the patient's tissue when the sensor isassembly is applied to a patient's tissue, and wherein the compressibleprotrusion has a compression modulus of less than about 2.0; and a cabledisposed on the substrate, wherein the cable is configured to be coupledto the monitor.
 13. The system, as set forth in claim 12, wherein thesensor assembly comprises a headband.
 14. The system, as set forth inclaim 12, wherein the compressible protrusion is configured to becompressed substantially flush with the tissue at an applied pressure ofgreater than 35 mm Hg.
 15. The system, as set forth in claim 12, whereinthe compressible protrusion is configured to be compressed substantiallyflush with the tissue at an applied pressure of greater than about 80 mmHg.
 16. The system, as set forth in claim 12, wherein the compressibleprotrusion is compressed to about 50% of its uncompressed height whenthe substrate is applied to the tissue.
 17. The system, as set forth inclaim 12, wherein the compressible protrusion is compressed to a heightof less than 2.5 mm when the substrate is applied to the tissue.
 18. Asensor comprising: a cap configured to be applied to a patient's head; asensor body disposed on the cap, and adapted to operate in a reflectancemode; an emitter disposed on the sensor body, wherein the emitter isconfigured to deliver a first light into a tissue; a detector disposedon the sensor body, wherein the detector is configured to detect thefirst light; and a compressible protrusion disposed on atissue-contacting surface of the substrate, wherein the compressibleprotrusion is configured to reduce the amount of a second lightimpinging the detector at an incident angle substantially in-line withan imaginary axis connecting the emitter and the detector, and whereinthe compressible protrusion has a compression modulus of less than about2.0.
 19. The sensor of claim 18, wherein the sensor comprises at leastone of a pulse oximetry sensor or a sensor for measuring a waterfraction.
 20. The sensor of claim 18, wherein the compressibleprotrusion is disposed in a region between the emitter and the detector.21. The sensor of claim 18, wherein the compressible protrusioncomprises, silicone, polyvinylchloride, a foam or a gel.
 22. The sensorof claim 18, wherein the compressible protrusion is configured to becompressed to 25% of its uncompressed height when the sensor is appliedto a patient.
 23. The sensor of claim 18, wherein the compressibleprotrusion is compressed from an uncompressed height greater than 5 mmto a compressed height of less than 2.5 mm when the sensor is applied toa patient.
 24. The sensor of claim 18, wherein the compressibleprotrusion is compressed from an uncompressed height greater than 2 mmto a compressed height of less than 1 mm when the sensor is applied to apatient.